1. The Field of the Invention
The present invention relates to a voltage protection circuit for a terminal of an electronic device. More particularly, the invention relates to a voltage protection circuit for a terminal of a linecard or any other communication apparatus.
2. The Relevant Technology
According to international norms, a linecard in a telecommunication system has to be protected against accidental application of a voltage, for example through a stroke of lightning or contact with a system voltage.
In FIG. 4, an example of a suitable voltage protection circuit is shown. A terminal line 1 of a linecard, for example the so-called TIP line of the linecard, is protected by an n-controlled thyristor 3. The anode side gate of the thyristor is biased with a positive voltage Vp, for example a positive battery voltage. One terminal of the thyristor 3 is coupled with the line 1 to be protected, another terminal is connected with mass or another suitable potential (“protective count”). If a positive voltage exceeding the gate voltage Vp is applied to line 1, the thyristor ignites, and the voltage applied on line 1 is deviated to mass. Thus, the linecard is protected from the high voltage.
For very short voltage pulses shorter than the gate turn on time of the thyristor, a capacitor (not shown) may be additionally provided to deviate these very short voltage pulses.
An extension of this concept to two lines to be protected (for example, the TIP and RING lines of a linecard) against both positive and negative voltages is shown in FIG. 5. The lines 1 and 2, for example the TIP and RING lines of a linecard, are both protected by two thyristors. Namely, each line is protected by an n-controlled thyristor 3 and with a positive gate voltage Vp for protection against positive voltages, as already discussed above. Additionally, a P-controlled thyristor 4 is provided for each of the lines 1 and 2. The cathode side gates of thyristors 4 are biased by a negative voltage Vn, for example a negative battery voltage. Similar to the thyristors 3, the thyristors 4 ignite if a negative voltage with an absolute magnitude exceeding the one of Vn is applied to the respective line 1 or 2. Thus, the linecard is also protected against a negative overvoltage.
In the following, as an example, only the protection against positive overvoltage by an n-controlled thyristor will be discussed. The concepts presented are likewise applicable to the case of negative overvoltage protection.
If a positive voltage exceeding Vp is applied to line 1 in FIG. 4 (or FIG. 5), a control current flows from the gate terminal of thyristor 3 to the voltage source providing Vp, for example a battery. However, on many linecards, the battery voltages are provided by a DC-DC converter. Such a DC-DC converter, because of its architecture, cannot accept a current. Thus, the above described control current cannot flow, and the potential applied at the gate of the thyristor increases along with a possible voltage increase at line 1, and consequently the thyristor does not ignite. In this case, the linecard is not protected against overvoltage.
Accordingly, additional elements have to be provided to deviate this current to mass. An example therefore is shown in FIG. 6, where a Darlington circuit with a bipolar transistor 5 is provided to meet that end. The transistor 5 has to be adapted such that it is in an on state when the voltage on line 1 exceeds the positive voltage Vp. Thus, a control current flowing from the gate terminal of thyristor 3 is deviated to mass.
With such an arrangement, two problems may occur. On the one hand, if the voltage Vp is high, for example 100 V, it is difficult to provide suitable transistors 5 as the maximum power dissipation or the maximum collector voltage is easily exceeded. On the other hand, the linecard may be part of a communication system where two signals are transmitted via the lines. An example for such a system is a combined standard telephone system (Plain Old Telephone System, POTS) and ADSL system (Asymmetric Digital Subscriber Line). When a call comes in, the voltage on the so-called TIP/RING lines exceeds the battery voltage as the voltage of a ring signal adds with the voltage of the ADSL signal. In the circuit shown in FIG. 6, in this case, the thyristor would ignite and limit the ADSL signal.